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76                                 MERCURIC CHLORIDE

MERCURIC   CHLORIDE (Corrosive Sublimate)

HgCl2 = 270-98 (271)

White crystals or crystalline masses soluble in about 16 parts of water
(at 15°), in 2-5 parts of 90% alcohol, or in 14 parts of ether. It may contain
as impurities, salts of sodium, manganese, zinc or other extraneous metals,
arsenic and calomel. The mercuric chloride, especially in basic preparations
of the salt, is determined as in 4.

1.  Impurities in general.—I grain, dissolved in 10 c.c. of water and
acidified with HCI, is treated with excess of hydrogen sulphide :   with the
pure, salt, a black precipitate and a colourless liquid are obtained, the latter
leaving no appreciable residue (alkali or alkaline earth salts, etc.) on evapora-

2.  Arsenic.—The sulphide precipitate from I is treated  with dilute
ammonia, and filtered ;  the filtrate should not give a yellow colour or pre-
cipitate on acidification with hydrochloric acid.

3.  Calomel.—i gram should dissolve completely in alcohol, or ether.

4.  Quantitative Determination.- -The estimation of mercuric chloride
in sublimate pastilles, cotton wool, gauze and other antiseptic preparations
is carried out as follows ;

(a) IODOMISTRIC METHOD. 2-3 grams of the sublimate or pastilles are
dissolved in water to 500 c.c. With cotton wool, gauze or other similar
material, which usually contains about 0*5% of the chloride, 20 25 grams
are digested for some hours with water, pressed out well and washed, the
liquid being filtered and made up to 500 c.c. An aliquot part of this solu-
tion, containing o-05-oao gram of sublimate, is acidified with a little hydro-
chloric acid and precipitated in the hot with hydrogen sulphide. The
mercuric sulphide is filtered off, washed and introduced, with the paper,
into a bottle with ground stopper ; a, little water (20-25 e.e.), about 5 <••<••
of carbon disulphidc and excess of N/io-iodiue (20 25 c.c. usually suffice)
are added, the bottle shaken vigorously and the excess of iodine titrated
with N/io-sodium thiosulphatc in presence of starch paste.1 The differ-
ence between the number of c.c. of iodine solution added and the number
of c.c. of thiosulphate necessary to act on the excess of iodine, gives, when
multiplied by 0-01356, the mercuric chloride in the volume of solution
taken for the determination.

(b) ALKAHMKIRIC OK HYDUAXINK MiCTiion,8 adapted especially for
analysis of corrosive sublimate pastilles or compresses. A pastille is dis-
solved in a little hot water and to the solution are added 20 c.c. of cold
saturated hydrazine sulphate: solution previously neutralised to methyl
orange, and exactly 10 c.c. of N-sodimn hydroxide ; the liquid is shaken,
left for a few minutes, and filtered, the filter being well washed with hot
water and the excess of sodium hydroxide in the filtrate titrated with N/io-

1  The mercuric sulphide reacts with  iodine  in potassium iodide solution, thus :
HgS -|- 21 "|-2,K,i; ----- 1'lgl.j -|  2.KI -|- S.    Carbon disulpludc is added in order to dissolve
the sulphur liberated in this reaction.

2  According to Rimini, l\'cntl. R. Ac-cad. Lined, XV, z, p, 32j, and Hull, chim. /arm,,

1908, p. 145.1) are treated in a beaker